Starting right now on ABC's "This week." Breaking overnight. Firefight. Ukrainian special forces clash with pro-russian militants. Both sides report casualties. Is this the start of the war? We're live on the ground with all of the breaking details. The contender. Are we supposed to consider you the front-runner now? On the road with Kentucky senator Rand Paul, how the rising star is shaking up the gop. And -- Boston strong. This is our City. Stay strong! One year later, a city facing new security fears, ready to rise again. From ABC news, "This week" with George stephanopoulos. Begins now. Good morning. Let's get right to that breaking news, a dangerous escalation of the crisis in Ukraine. There are reports of new casualties and the U.S. Is warning Russia to stand down now. U.n. Ambassador Samantha power is here live with an exclusive interview after this report from ABC's Alex Marquardt on the scene in Ukraine. Reporter: As you just said, this is a dangerous escalation in eastern Ukraine this morning. They have been deployed to the city of slovansk, as part of an antiterrorist operation, aimed against armed pro-russia forces. That yesterday seized government buildings including the police headquarters. There have been dead and wounded on both sides. At least one Ukrainian officer has been shot and killed. So far, the Ukrainian forces haven't been able to take back the buildings. We're about ten miles outside of sloviansk, on our way there. Just passing through many of the new checkpoints that have sprout up around town, manned by activists waving pro-russia signs. Across eastern Ukraine for the past week, we have seen buildings violently seized by antigovernment protesters. The U.S. Has directly accused Russia of being behind this new wave of unrest which Moscow has denied. Washington says that many of the armed men that we have seen seizing these buildings over the past 24 hours, have the very same uniforms and guns that they had when Russian forces annexed crimea last month. Violence against Russian speakers here would provoke a response. And with around 40,000 Russian troops, as well as fighter jets, lining that border with eastern Ukraine, the new fear is now that this new violence will give Russia a new reason to invade. Let's get more from Martha Raddatz. Secretary Kerry immediately on the phone with his counterpart. Exactly, George, this is the kind of encounter U.S. Officials feared could ignite greater controversy. Strong concerns that attacks by armed militants in eastern Ukraine were orchestrated and synchronized similar to previous attacks in crimea. There will be additional consequences if Russia doesn't take steps to back down. They clearly believe that this is orchestrated by Russia. It could give Russia an excuse to move in and just a short time ago, George, U.N. Secretary-general ban ki-moon urging maximum restraint. Of course the big question, what more could the U.S. Too? We also know that vice president Biden is heading there last week. There's been a report out of the Ukrainian parliament that cia director John bremen is there. You can try to isolate Russia and support Ukraine military with food and supplies, but that, too, can escalate it. Biden is heading over for meetings with the Ukrainians and the cia is not officially confirming the visit, the Russian news agencies claims that bremen slipped into kiev using a false name and gave his blessing to the Ukrainian forces which is highly suspect. As you said, the U.S. Is not denying not confirming. Thank you very much.

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